As compared with widely used amorphous silicon (a-Si), amorphous (non-crystalline) oxide semiconductors have high carrier mobility (also called as field-effect mobility, which may hereinafter be referred to simply as “mobility”), high optical band gaps, and film formability at low temperatures, and therefore, have highly been expected to be applied for next generation displays, which are required to have large sizes, high resolution, and high-speed drives; resin substrates having low heat resistance; and others.
In the oxide semiconductors, amorphous oxide semiconductors consisting of indium, gallium, zinc and oxygen (In—Ga—Zn—O, which may hereinafter be referred to as “IGZO”) have preferably been used, in particular, because of their having extremely high carrier mobility. For example, non-patent literature documents 1 and 2 disclose thin-film transistors (TFTs) in which a thin-film of an oxide semiconductor having an In:Ga:Zn ratio equal to 1.1:1.1:0.9 (atomic % ratio) was used as a semiconductor layer (active layer). In addition, patent document 1 discloses an amorphous oxide containing In, Zn, Sn, Ga and other elements, as well as Mo, in which the atomic composition ratio of Mo, relative to the number of all the metal atoms in the amorphous oxide, is from 0.1 to 5 atomic %. Examples of patent document 1 disclose TFTs using an active layer formed by addition of Mo to IGZO.
When an oxide semiconductor is used as a semiconductor layer of a thin-film transistor, the oxide semiconductor is required to have a high carrier concentration (mobility) and excellent TFT switching characteristics (transistor characteristics or TFT characteristics). More specifically, the oxide semiconductor is required to have, for example, (1) a high on-state current (i.e., the maximum drain current when a positive voltage is applied to both a gate electrode and a drain electrode); (2) a low off-state current (i.e., a drain current when a negative voltage is applied to the gate electrode and a positive voltage is applied to the drain voltage, respectively); (3) a low S value (Subthreshold Swing, i.e., a gate voltage needed to increase the drain current by one digit); (4) a stable threshold value (i.e., a voltage at which the drain current starts to flow when a positive voltage is applied to the drain electrode and either a positive voltage or a negative voltage is applied to the gate voltage, which voltage may also be called as a threshold voltage) showing no change with time (which means that the threshold voltage is uniform in the substrate surface); and (5) a high mobility.
Furthermore, TFTs using IGZO or other oxide semiconductor layers are required to have excellent resistance to stress such as voltage application and light irradiation (stress resistance). It is pointed out that, for example, when a voltage is continuously applied to the gate electrode or when light in a blue emitting band in which light absorption starts is continuously irradiated, charges are trapped on the boundary between the gate insulator layer and the semiconductor layer of a thin-film transistor, resulting in a variation of switching characteristics, such as a shift of the threshold voltage. In addition, for example, when a liquid crystal panel is driven or when a negative bias is applied to the gate electrode to turn on a pixel, the TFT is irradiated with light leaked out from the liquid crystal cell, and this light gives stress to the TFT to cause a deterioration in the characteristics. Indeed, when a thin-film transistor is used, a variation of switching characteristics due to stress by voltage application causes a lowering of reliability in a display devices itself, such as a liquid crystal display or an organic EL display. For example, a variation of switching characteristics in an organic EL display creates a need to flow a current of several μA or higher for driving an organic EL element. Therefore, an improvement in the stress resistance (a small variation before and after the stress tests) has eagerly been desired.
The above-described deterioration in the TFT characteristics by stress such as voltage application or light irradiation is due to the formation of defects in the semiconductor itself or on the boundary between the semiconductor and the gate insulator layer during the stress tests. As the gate insulator layer, insulators such as SiO2, Si3N4, Al2O3 and HfO2 are widely used in ordinary cases, but the boundary between the semiconductor layer and the insulator layer is an area where different materials come into contact with each other, and therefore, it is considered that defects are particularly liable to be formed. To improve stress resistance, the handling of this boundary between the semiconductor layer and the insulator layer seems to be particularly very important.
To solve the above problems, for example, patent document 2 discloses a method of controlling defects in grain boundaries to improve stability by the use of an amorphous oxide of In-M-Zn (M contains at least one of Ga, Al, Fe, Sn, Mg, Cu, Ge and Si) for a gate insulator layer. However, there is the possibility that the use of a method according to this document causes an increase of defects on the boundary between the gate insulator layer and the semiconductor layer, thereby lowering stability, because the gate insulator layer contains In liable to form oxygen defects.